docs: document JS SDK installation (#9611)

- Add a page introducing JS SDK + how to install and use it (generally)
- Adjust admin tips on how to send requests
- Adjust storefront tips to mention JS SDK
- Add in the API reference intro how to install JS SDK
- Other related additions / changes

Closes DX-957

www/apps/resources/generated/files-map.mjs

@@ -595,6 +595,10 @@ export const filesMap = [
     "filePath": "/www/apps/resources/app/integrations/page.mdx",
     "pathname": "/integrations"
   },
+  {
+    "filePath": "/www/apps/resources/app/js-sdk/page.mdx",
+    "pathname": "/js-sdk"
+  },
   {
     "filePath": "/www/apps/resources/app/medusa-cli/commands/build/page.mdx",
     "pathname": "/medusa-cli/commands/build"